This invention relates to a pretreated painted metal with a composite coating for forming an adherent bond between the paint and the metal substrate. More particularly, the invention relates to a two step process for pretreating the metal with a composite coating including an inner inorganic layer and a siloxane containing outer organic layer.
It has been proposed to improve corrosion resistance and paint adhesion on cold-rolled and galvanized steel by rinsing with a solution containing an organic polymer and a silane. It also has been proposed to rinse the steel with a solution containing a silane and a silicate dispersion for this purpose as well. U.S. Pat. No. 5,108,793; incorporated herein by reference, relates to forming a painted steel pretreated with a two step silane treated silicate coating. The coating is formed by rinsing the steel in an alkaline solution having a temperature of at least 25.degree. C. containing 0.005M silicate and 0.005M metal salt. The steel is dried to form a silicate coating having a thickness of at least 20 .ANG. prior to being treated with an aqueous solution containing 0.5-5 vol. % silane. The silane layer forms a relatively adherent bond between the paint and the silicate coating.
Unlike cold-rolled steel, it is very difficult to obtain good paint adhesion on chromium alloyed steels such as stainless steel. An apparent reason for poor paint wettability is because the surface of these steels is covered with a passive oxide. Many different types of etchants have been used to remove this passive oxide to enhance paint adhesion. This approach, however, has not resulted in good paint adhesion on stainless steel exposed to a long term humid environment.
Other artisans have proposed one or two step rinsing techniques for stainless steels using an acid and a silane coupling agent prior to painting. For example after being rinsed with chromic acid, the etched steel may be rinsed with an aqueous solution containing the silane. Alternatively, the acid solution may include a dispersion of a silicate and/or a dissolved silane.
Still other artisans have tried pretreating stainless steel with alkyl or fluoroalkyl oxide coatings prior to painting. The inorganic coating is formed by rinsing the steel with a solution containing a dispersion of oxide particles of one or more of silicon, aluminum, zirconium, titanium, cerium or tungsten. The solution also may include a silane.
Although chromate films improve paint adhesion on stainless steel, chromic acid solutions or waste materials are not environmentally acceptable and may not be economically disposed of. When used in a one step pretreatment process, a silane does not have the molecular orientation necessary for good paint adhesion. Consequently, silane generally is used in concentrations that result in a coating having an excess thickness. A thick coating, however, tends to be brittle having poor formability. On the other hand, an inorganic coating formed from a solution containing a dispersion of oxide particles is disadvantageous because the liquid and solid phases are incompatible and form a mechanically weak coating structure.
As evidenced by the effort of previous workers, there has been a long felt need to develop a process for improving paint adherence to metal, especially stainless steel. The process should be low cost, use nontoxic materials that can be safely disposed of and provide long term resistance in a humid environment.